The invention is generally directed to a magneto-optical signal read-out system for a magneto-optical player and in particular to a magneto-optical signal read-out system with significantly improved signal to noise ratios resulting from elimination of in-phase noise by use of a differential amplifier based system.
In general there are two distinct types of magneto-optical players: the reflection type, in which the information is read out by utilizing the Kerr rotation angle; and the transmission type utilizing the Faraday rotation angle. An example of the reflection type magneto-optical player is disclosed in the Japanese publication of the Institute of Electrical Engineers in Japan, Magnetics, MAG-84-70.
Reference is made to FIG. 1 wherein a conventional optical head of the type used in a reflection type magneto-optical player is shown. The differential read-out system enclosed by a dash line, generally indicated as 120, is described in detail. The light reflected by magneto-optical disk 106 first passes through a half-wave plate 100 and the polarized plane of the light is rotated by 90.degree.. Light is then split into transmitted light and reflected light by a beam splitter 101. The light then passes through analyzers whose analyzing, axes with respect to the polarized plane of the incident light are .+-.45.degree.. The reflected light is then condensed by lens 102 and focused upon photodiode 103. Similarly, the transmitted light passes through and is condensed by lens 104 and is focused upon photodiode 105.
A differential amplifier takes the difference between the outputs from photodiodes 103 and 105 to detect the signal. By use of this differential detection method, in-phase noise such as laser noise is eliminated. Accordingly, the signal to noise ratio (S/N) of the reproduced signal is improved. As a result, the differential detection method is generally used in optical heads for magneto-optical recording.
An example of a transmission type magneto-optical player is described in detail in publication entitled PROCEEDINGS OF SPIE, Volume 382, pp. 240-244 (1983). The structure of transmission type optical head is shown in FIG. 2. In FIG. 2, the light transmitted through the magneto-optical disk 107 passes through the analyzer 108 to a 4-divided photodiode 109 (the terminology n-divided photodiode will be used in this application to describe a photodiode which is divided into n-parts (where "n" is an integer greater than 1) as a shorthand reference term). This 4-divided photodiode 109 consists of 4 similar photodiodes coupled in parallel. The photodiodes effect the focusing and tracking and permit the RF signal (magneto-optical signal) to be obtained at the same time. In this example, the differential detection method to eliminate in-phase noise as described above is not utilized. For purposes of focusing and tracking, the light source side and the photodetector sides of the optical head are driven together as one body in this example.
A transmission type optical head recently developed by the applicants and now the subject of pending application No. 06/925,202, filed Oct. 31, 1986, is shown in FIG. 3. The principle of the system of FIG. 3 is that the tracking and focusing are performed using the reflected light and the RF signal is detected using the transmitted light. In FIG. 3, the tracking and focusing with respect to magneto-optical disk 110 are controlled by the two dimensional actuator 116 of the objective lens, beam splitter 111, knife-edge 112 and the 4-divided photodiode 113. The RF signal is detected by the analyzer 114 and the photodiode 115.
The magneto-optical system of FIG. 3 is an improvement over the system depicted in FIG. 2 because track accessing is performed in two stages. The tracking system includes a coarse accessing adjustment and a fine accessing adjustment which improves the tracking accuracy and shortens the access time.
In reflection type optical heads, as shown in FIG. 1, the optical signal is generally detected by the differential detection method. Thus, a half-wave plate, a polarizing beam splitter, two lenses and two photodiodes are necessary to implement the optical head. This results in an optical head which inevitably is large, heavy and expensive, all of which are disadvantageous.
Transmission type optical heads have failed to utilize a differential detection method for optical signal detection. The transmission type optical heads are arranged so that structures are located both above and below the magneto-optical disk which causes the head to be large and heavy. If a differential detection arrangement, as is shown in broken line 120 in FIG. 1, is provided at the light transmission side of the optical head, the head becomes much larger, heavier, slower and more expensive, all of which characteristics are undesirable.
A large optical head provides a limit on the miniaturization of the magneto-optical player which is an ongoing design goal. A heavy optical head decreases speed of movement and prevents high-speed access increasingly required in new applications of magneto-optical storage media.
Accordingly, there is a need for an improved magneto-optical head with improved performance utilizing a differential detection method, which is small, lightweight and inexpensive.